A recent development in the synthesis of peptides is the solid phase technique developed by Merrifield [JACS, 85 2149, (1963)] . In this process, the C-terminal amino acid of the projected peptide, in protected form, is bonded to an insoluble polymer.
This bonding is effected by reacting a chloromethylated polymer in the presence of an organic solvent such as ethanol or ethyl acetate with the triethylammonium salt of the protected amino acid. This reaction takes place at reflux temperature and requires two days to form the desired protected amino acid benzyl ester in maximum yield.
From the ester so formed, the amino protecting group is removed in a manner well known to those skilled in the art, for instance, by treatment with hydrochloric or hydrobromic acid in acetic acid, or with trifluoroacetic acid.
The amino group thereby liberated is acylated with another protected amino acid, and the previous sequence of steps, that is, de-protection and acylation is then repeated until the desired peptide has been attached to the polymer. The peptide is then de-protected and removed from the polymer, e.g., by saponification.
The foregoing process is subject to certain drawbacks; (1) it is necessary to perfrom the esterification step under such relatively drastic conditions that some undesired side reactions, such as racemization of the amino acid is a distinct possibility; (2) an undesirable side reaction, namely the formation of the quaternary ammonium salt from the chloromethyl polymer and triethylamine, occurs during the esterification step; and (3) the conditions required in the esterification step for the attachment of the amino acid peptide to the solid polymeric support are unlike those required in the acylation steps for the attachment of all the remaining amino acids of peptide chain. This difference is a disadvantage in the above method because it precludes the carrying-out of both the esterification and acylation steps in the same reaction vessel, i.e., the method is not totally automated.